Mathematically speaking the way it's written would be interpreted as ax^2 since the power takes precedence over the multiplication, not (ax)^2. So it's fine to say 2.5mm2. If you wanted 2.5 x 2.5 you would have to make that explicit.
Things could easily have changed but early panel cabling was expected to carry the maximum short circuit current a panel array could produce without hazard.
2.5 millimetres squared is a perfectly valid measure for something of
*any* shape with a cross sectional area equal to that of a square measuring 2,5mm on each side but this is a considerably larger area than that of a conductor in a 2.5mm^2 T&E cable conventionally used for a 30 amp ring main. And for educators (e.g. the guys on Efixx) to use this term is wrong, they should say "square millimetres".
The square root of 2.5 is 1.58 so a 2.5mm^2 conductor has an area equal to 1.58 millimetres squared.
The NPL defines the SI symbol for the basic derived unit of area as m^2 with the name 'square metre' and *not* 'metre squared'
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